Файл: Antsiferov V.V., Smirnov G.I. Physics of solid-state lasers (ISBN 1898326177) (CISP, 2005)(179s).pdf

Solid-state neodymium lasers in free lasing regime

Table 2.1

Here λ 0 is the wavelength of lasing in the centre of the gain line; ∆λ is the width of the gain line; τ is the lifetime of the upper working level; σ is the cross-section of the induced transition.

Irradiation of Nd glass with pumping radiation with a wavelength shorter than 400 nm results in the formation of dynamic dye centres in these crystals associated with the matrix of the glass and showing absorption at the lasing wavelength. This results in the losses and modulation of radiation intensity [41]. The line structure of the lasing spectrum, observed in Nd glass lasers is determined by the presence of the spurious selection of longitudinal modes in the resonator [42].

The parameters, optical homogeneity and transparency (the absorption losses are of the order of 10–3 cm–1) of the Nd glass laser are considerably superior to those of the artificial crystals with Nd. The dimensions of the active elements made of glass are restricted by the obtainable pumping parameters and not by technology, as in the case of crystals. Nd glass does not contain metallic impurities and is characterised by the highest beam resistance to radiation. The failure threshold of glass is 103 104 J/cm2 in the free lasing regime, 102 103 J/cm2 in the regime of the giant pulse, and 1 10 J/cm2 in the regime of supershort radiation pulses.

Both silicate and phosphate glasses are used in practice. The silicate glasses have higher technological properties and high mechanical strength and chemical resistance but they are characterised by a smaller cross-section of the induced transition and a higher non-linear refractive

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index. This limits the applicability of these materials in powerful amplifiers. The phosphate glasses are characterised by higher efficiency, lower lasing threshold, a narrower radiation spectrum, a higher threshold of non-linear effects and better thermo-optical characteristics. A significant disadvantage of the phosphate glasses is their low mechanical strength and high hygroscopicity. Consequently, the ends of active elements require special protection.

The main physical and spectral characteristics of the investigated phosphate glasses GLS-22 were as follows: the concentration of Nd ions 1.6% (2.0 × 10 20 cm–3); density 3.5 cm g/cm3; refractive index 1.58; heat conductivity 0.4 W/m deg; absorption factor 2 × 10 –3 cm–1; the coefficient of linear expansion 10.6 × 10 –6 deg–1; the thermo-optical coefficient –5.7 × 10 –6 deg–1; quantum yield 0.6–0.8. The maxima of the wavelengths (absorption factors) of the most intensive absorption bands were 512 nm (0.45 cm–1), 525 nm (0.77 cm–1), 585 nm (2.67 cm–1), 746 nm (1.47 cm–1), 804 nm (1.85 cm–1) and 804 n (0.47 cm–1). The maximum of the wavelength of luminescence was 1055 nm, the maximum of the lasing wavelength 1054 nm, the lifetime of the upper working level 300

µs, the transitions section 4F3/2 → 4I11/2 3.5 × 10 cm 2, the width of the gain line 19 nm (effective width 28 nm).

The efficiency of lasing of glass lasers can be increased by increasing the concentration of active ions. However, at a Nd ions concentration of ≥ 3% (2 × 10 20 cm–3), the majority of active media are characterised by a decrease of the quantum yield and the efficiency of lasing as a result of the concentrational decay of luminescence (CDL). CDL greatly intensifies in cases in which the distance between the adjacent Nd ions decreases to 0.3 nm.

In the dipole–dipole interaction, the probability of decay is proportional to R–6, where R is the distance between the interacting particles. Therefore, the increase of the distance between the adjacent ions by a factor 1.5 results in a large decrease of the CDL with the concentration of the active ions increasing by almost an order of magnitude. For example, for Li– Nd–La phosphate glass (KNFS) with the minimum distance between the adjacent Nd ions of 0.47 nm, the concentration of the active ions may increase to 3 × 10 21 cm–3 without any large decrease of the quantum yield [31]. The application of KNFS glass in practice is restricted by the extremely high hygroscopicity of this material, low hardness and heat conductivity.

The main and spectroscopic characteristics of the examined KNFS glasses: the concentration of Nd ions 4 and 8 × 10 20 cm–3; density 2.85g/ cm3; refractive index 1.55; the coefficient of linear expansion 0.8 × 10–6 deg–1; the thermo-optical coefficient 1.3 × 10 –6 deg–1; the maximum of the lasing wavelength 1054 nm; the section of the 4F3/2 → 4I11/2 transition

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Solid-state neodymium lasers in free lasing regime

3.8 × 10 –20 cm2; the lifetime of the upper level is changed from 330 to 80 µs with the change of the concentration of the Nd ions from 4 to 30 × 10 20

2.2.1 Spectral–time lasing parameters

The spectral–time, angular and energy characteristics of the free lasing of Nd lasers in different glasses were investigated by the authors of this book in Ref. 1,3,4,9,11,18,19,22–28.

In the Nd glass lasers with spherical mirrors, the lasing of TEMmnq modes takes place in the regime of non-attenuating and non-regular pulsations of radiation intensity in all cases, with the exception of the case in which the configuration of the resonator is confocal or concentric (critical). When smoothing the spatial heterogeneity of the field in the active medium by compensated phase modulation (CPM) in a wide range of the radiation of the parameters of the spherical resonator or in the case of excitation of a large number of degenerate waves in resonators with critical configuration, it is always possible to achieve quasistationary lasing.

In a laser with a flat resonator with optimum length (L ~ 2 m) and a low value of the indices of the transverse modes (m, n ≤ 5), the lasing of TEMmnq modes takes place in the quasistationary regime. The spatial competition of these modes increases with increasing values of the indices of the transverse waves. This competition leads to the alternation of the transverse (Fig. 2.2b) and longitudinal modes (Fig. 2.2c) during the lasing process and the formation of non-attenuating pulsations of radiation intensity. In this case it is not possible to stabilise the radiation wavelength when using a complex dispersion resonator (Fig. 2.2d) [3,4].

The quasistationary lasing of TEMooq modes in a Nd glass laser that was stable in a wide pumping range (Fig. 2.3) [3,4] was obtained with the optimum parameters of the flat resonator (length 2 m, diaphragm diameter 2 mm). This was carried out by dynamic tuning of the elements of the resonators. The procedure used in this case was described previously. In contrast to Cr lasers, in the Nd lasers it is not necessary to smoothout the longitudinal heterogeneity of the field in the active medium. The silicate glass is characterised by fork splitting of the spectrum during the lasing process (Fig. 2.3c), determined by the non-uniform nature of broadening of the gain line. At low Nd ion concentrations (~ 1%) phosphate glass did not show such splitting of the lasing spectrum (Fig. 2.3d) because of the higher rate of transfer of excitation energy inside the non-uniformly broadened line. Increase of the concentration of the Nd ions in the phosphate glass and the KNFS time glass resulted in the splitting of the lasing spectrum with a finer structure (Fig. 2.3e). When using the complex dispersion resonator (with dispersion prisms

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Fig.2.2 Parameters of the lasing of TEMmnq modes of a Nd laser on silicate glass (diameter 8 mm, length 130 mm) with flat mirrors (L = 2 mm) without selection of longitudinal modes (a–c) and with selection (d), Ep = 2Et; a) the oscillogram of the intensity of radiation, 20 µs marks; b) the time evolvement of the distribution of radiation intensity in the near-zone; c,d) time evolvement of the radiation spectrum from DFS-8 spectrograph (c) Fabry–Perot interferometer, dispersion range 20 pm.

Fig.2.3 Parameters of the lasing of TEM ooq modes of neodymium laser in silicate glass (8 mm diameter, 130 mm length) phosphate (diameter 6.3 mm, length 120 mm) and concentrated phosphate glass (6.3 mm diameter, length 100 mm) with flat mirrors (L = 2 m, diameter 2 m) with elimination of the effect of technical interference of the resonator without selection of the longitudinal modes (a–e) and with selection (f), Ep = 4Et; a) oscillogram of radiation intensity, 20 µs marks; b) time evolvement of the distribution of intensity in the near-range zone; e–f) time evolvement of the lasing spectrum from DFS-8 spectrograph (c–e) Fabry–Perot interferometer (f), the region of dispersion of the interferometer 20 pm.

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Solid-state neodymium lasers in free lasing regime

Fig.2.4 Interferogram of the integral spectrum of radiation of TEMooq modes of the neodymium laser in glass in quasi-stationary regime with selection of longitudinal modes (a), Ep = 4Et; b) sequence of radiation spectrographs, illustrating the range of rearrangement of the wavelength of lasing of the Nd glass laser.

and the Fabry–Perot selector-etalon) it was possible to obtained stable single-frequency quasistationary lasing of the TEMooq modes in all types of glass (Fig. 2.3f). In a this case, the width of the integral radiation spectrum was approximately 0.3 pm (Fig. 2.4 a), and the width of the rearrangement range was 30 nm (Fig. 2.4b) [11].

2.2.2 Energy parameters of lasing

Experiments were carried out using standard active elements of silicate glass (LGS-250) and phosphate glass (GLS-22) with the ends cut under an angle of 1º and illuminated ends, with the concentration of the Nd ions of 2 × 10 20 cm–3, the dimensions: diameter 6.3 × 110 mm. The volumes of the active media, contributing to the lasing energy, were Vg = 2.5 cm3. Li–Nd–La phosphate glass (KNFS) was characterised by the concentration of Nd ions of 4 × 10 20 cm–3, diameter 6.3 mm, 75 mm length, with Vg = 2 cm3. Pumping was carried out using an IFP-800 lamp in a quartz single-block illuminator doped with europium, with the pumping pulse time of 250 µs. Liquid filters were used for additional cutting off of ultraviolet pumping radiation.

Of all the active media with the Nd ions, examined in these investigations, the lowest efficiency was recorded in the case of silicate glass characterised by the lowest value of the laser transition cross-section. Therefore, the lowest energy characteristics of radiation were obtained in the case of silicate glasses, the highest for the concentrated phosphate glasses (KNFS). With increase of the length of the flat resonator and a constant energy of pumping, the lasing energy of the lasers on glass decreased (Fig. 2.29a) (4,6), as in the majority of active media. However, if the lasing energy of the silicate and phosphate (6) glasses was halved when the length was increased from 0.3 to 1.6 m, the lasing

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energy of the KNFS glass (4) decreased 4 times. When heating the silicate and phosphate glasses from 10 to 90 °C, the lasing energy decreased almost linearly by a factor of 1.2 (Fig. 2.29b) (6). For the KNFS glass, the temperature dependence could not be recorded because when heating this type of glass microcracks appeared as a result of thermal deformation of the glass. At high pumping energy, these microcracks resulted in failure of the active elements.

For every pumping level there is the optimum coefficient of transmission of the output mirror of the resonator at which the energy of lasing is maximum. At a pumping energy of 300 J the optimum transmission factor for the silicate and phosphate glasses was approximately 35%, and for the KNFS glass it was 50% (Fig. 2.29c) (4,6). As indicated by Fig. 2.30a (4,6), with increase of the pumping energy the values of the optimum factors reach saturation approaching 40 and 60%, respectively. At a low pumping energy, the increase of the lasing energy is linear (Fig. 2.29d) (4,6). The linear dependence was already violated at pumping energies of approximately 100 J.

If the ratio of the lasing energies of the lasers on KNFS and GLS22 at low pumping energy level was 2, then at a pumping energy of 500 J this ratio was almost halved. This was determined by higher thermaloptical deformation of KNFS glass which failed at high pumping energy. At a pumping energy of 500 J, the density of the lasing energy of the lasers was Eg/Vg = 0.8 J/cm3 for GLS-22 and 1.6 J/cm3 for KNFS. At low pumping energy levels, the divergence of the radiation of the two lasers was almost identical, and with increase of the pumping energy, the divergence of radiation of the laser on KNFS increased more rapidly (Fig. 2.30b).

2.3 Nd:YAG LASERS

Of the entire range of the crystals, the crystals of yttrium–aluminium garnet with neodymium, Nd3+:Y3Al5O12(Nd:YAG), are used most widely. These crystals are characterised by a large number of unique properties and are capable of operating in all possible lasing regimes. The Nd:YAG crystals have the cubic symmetry and are characterised by relatively high values of heat conductivity (14 W/m deg) and Moose hardness (8.5). The density of the garnet crystals is 5.04 g/cm3, melting point 1930 °C, the thermal expansion coefficient 6.96 × 10 –6 deg–1, and the thermo-optical constant dn/dT = 9.86 × 10 –6 deg–1 [32]. The transparency region of the garnet crystal is 0.24–6.0 µm. The Nd:YAG crystals are characterised by high resistance to optical radiation, a low excitation threshold and their service life is almost unlimited. Mean powers of continuous lasing of up to several kW have been obtained in the case of Nd:YAG lasers.

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